spi: Add support for mspi on brcmstb SoCs

Added documentation for compatibility for brcmstb SoCs :
7425, 7429, 7435, 7216, 7278

Signed-off-by: Kamal Dasu <kdasu.kdev@gmail.com>
Link: https://lore.kernel.org/r/20200420190853.45614-3-kdasu.kdev@gmail.com


Signed-off-by: Mark Brown <broonie@kernel.org>